Optical position-measuring devices include a measuring standard and a scanning unit moveable relative thereto. The scanning unit typically contains a light source, optoelectronic detector elements, as well as additional optical components such as lenses, gratings, etc. However, a scanning unit having all these components may be too large in size for certain applications. Furthermore, in certain applications, the active components of such a scanning unit may be undesirably affected by electromagnetic interference fields or increased temperatures. Moreover, electrical power dissipated by the light source may cause a thermal effect on the measurement accuracy.
Therefore, solutions for optical position measuring devices have become known, in which only the scanned measuring standard as well as a passive fiber optic scanning head movable relative thereto are disposed at the actual measurement location. The other active components that are relevant for the scanning operation, such as the light source and optoelectronic detector elements, are disposed at a distance from the measurement location and connected to the fiber optic scanning head by optical fibers. In this way, it is possible to achieve an extremely compact system at the actual measurement location. Moreover, it is also possible to minimize interference effects on the various active components on the scanning side.
A position-measuring device of this type has been proposed, for example, in the Applicant's German Patent Application DE 10 2007 024 349 A1. The position-measuring device disclosed in this publication includes a measuring standard in the form of a reflection measuring standard and, in addition, a fiber optic scanning head movable relative thereto in at least one measuring direction. Additional components of the position-measuring device are disposed downstream of the fiber optic scanning head at a distance therefrom and connected by a suitable optical fiber. These components include, for example, the light source required for optical scanning, a plurality of optoelectronic detector elements, and possibly further active components for signal generation and processing. The purely passive fiber optic scanning head includes substantially a scanning reticle and the end of an optical fiber for delivering illumination and for returning the signal-generating beams of light. In order to generate a plurality of phase-shifted scanning signals, provision is made for wavelength-dependent coding of the scanning signals. To this end, the scanning reticle is illuminated with broadband light, and the scanning reticle is formed by a plurality of elementary cells, each composed of a plurality of regions which are transparent to different wavelength ranges of the illumination spectrum. To this end, the different regions of each elementary cell of the scanning reticle are designed, for example, as partial etalons having different transmission wavelengths. The scanning principle used in DE 10 2007 024 349 A1, however, cannot be used in conjunction with reflection measuring standards having very small grating periods. The reason for this is that the size of partial etalons cannot be made arbitrarily small while at the same time retaining their wavelength-dependent transmission.